DC to DC converter and power management system

ABSTRACT

A DC to DC Converter includes an electrical circuit that allows batteries and other electrical energy storage devices to be charged from or to discharge to a variable voltage DC bus. This electrical circuit also enables seamless integration with other energy storage devices and/or DC power sources, such as fuel cells, to provide DC power for a Power Management System. A Power Management System preferably provides both full power source management and power conditioning. The Power Management System is able to manage power flow to and from multiple, isolated power sources and energy storage devices to deliver high quality alternating current (“AC”) power to a load.

This application claims priority from U.S. Provisional PatentApplication Serial No. 60/221,596, filed Jul. 28, 2000.

BACKGROUND OF THE INVENTION

This invention relates generally to systems and methods for managingdirect current (“DC”) power. More specifically, this invention relatesto DC to DC converters and power management systems and methods.

Today, most hybrid fuel cell/battery power systems, and other systemshaving multiple DC power sources and batteries, are arranged as shown inFIG. 1. The arrangement shown in FIG. 1 is referred to as the “batterynode” approach because the power must pass through the battery outputnode at the battery voltage. This configuration therefore uses batterycharge controller and inverter ratings that match the capacity of thefuel cell.

Conventional DC to DC converters and their associated inverter designsand products have several deficiencies that make it difficult for themto adequately meet the functional requirements of modern hybrid powersystems. These conventional converters are therefore unable to satisfythe needs of a typical energy user. Among other problems, conventionalDC to DC converters typically generate electrical noise and highfrequency ripple currents on the input (source) and output (load)busses. They are also poorly adapted to the regulation of input current.Furthermore, they typically exhibit energy conversion efficiencies ofonly around 80-90%.

SUMMARY OF THE INVENTION

According to one aspect of this invention, a DC to DC Buck and BoostConverter is provided. “Buck” power conversion refers to a reduction involtage from an input side of the converter to an output side. “Boost”power conversion refers to an increase in voltage from the input side tothe output side of the converter. According to one embodiment of thisinvention, the Buck and Boost DC to DC Converter includes an electricalcircuit that allows batteries and other electrical energy storagedevices to be charged from or to discharge to a variable voltage DC bus.This electrical circuit can also be configured to enable seamlessintegration with other energy storage devices and/or DC power sources,such as fuel cells, to provide DC power for a Power Management System.

Improved Boost DC to DC Converters are also provided which reduce noiseand ripple currents in low voltage/high current applications. Accordingto one embodiment, a resonant capacitance is provided by two resonantcapacitors which store voltage using switches that permit zero voltageswitching. According to another embodiment, an input capacitor isprovided to maintain a constant voltage input to a resonant circuit. Theaddition of the input capacitor reduces voltage stress in a switchingcircuit.

A DC to DC Converter is provided in a module of a Power ManagementSystem. The Power Management System preferably provides both full powersource management and power conditioning. In other words, the PowerManagement System preferably manages power flow to and from multiple,isolated DC power sources and energy storage devices, while deliveringhigh quality alternating current (“AC”) power to a load.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features and advantages of the present invention willbecome more readily apparent from the following detailed description,made with reference to the following figures, in which:

FIG. 1 is a schematic block diagram of a conventional “battery node”hybrid power configuration according to the prior art.

FIG. 2 is a schematic block diagram of a series power management systemconfiguration according to one embodiment of the present invention.

FIG. 3 is a schematic block diagram of a parallel power managementsystem according to another embodiment of the present invention.

FIG. 4 is a schematic circuit diagram of a Buck and Boost DC to DCConverter according to another aspect of this invention.

FIG. 5A is a schematic circuit diagram of a Boost DC to DC Converteraccording to a further aspect of this invention.

FIG. 5B is a schematic circuit diagram of a Boost DC to DC Converteraccording to a still further aspect of this invention.

FIG. 5C is a schematic circuit diagram of a Boost DC to DC Converteraccording to the prior art.

FIG. 6 is a graph illustrating the current flow over time through aninductor of the Buck and Boost DC to DC converter of FIG. 4.

DETAILED DESCRIPTION

There are presently two preferred Power Management System configurationsaccording to this invention, including a series converter configuration,illustrated in FIG. 2, and a parallel converter configuration, shown inFIG. 3. In FIGS. 2 and 3, the hybrid power systems 10, 110 of these twoembodiments are shown having only a fuel cell 20 and a battery bank 25.It should be noted, however, that the fuel cell 20 could easily bereplaced by any other DC power source, by a rectified AC source, or byan energy storage unit, as desired. Similarly, the battery bank 25 couldbe replaced by a flywheel energy storage unit or other energy storagedevice that is charged with and discharges direct current. It shouldalso be noted that any number of fuel cells 20 and DC to DC Converters14 can be arranged in parallel to supply current to the inverter. Andfurthermore, the single battery bank 25 could be replaced by any numberof parallel battery banks or strings connecting to the common bus.

As noted above, the schematic block diagrams in FIGS. 2 and 3 illustratea series and a parallel Power Management System 10, 110, respectively,according to preferred embodiments of the present invention. Referringto FIGS. 2 and 3, each of the Power Management Systems 10, 110 includesthree main modules: the Controller Module 12; the DC to DC ConverterModule (including the current control circuit) 14, 114; and the InverterModule 16. Any number of independent power sources, such as fuel cell 20and battery bank 25, are also included.

The independent power sources 20, 25 can operate over different voltageranges. The Controller Module 12 senses and analyzes the operatingoutput of the power sources 20, 25. The DC to DC Converter Module 14,114 in conjunction with the Controller Module 12, manages andconsolidates the power from these sources 20, 25 into a DC bus 13 forprocessing by the Inverter Module 16. The system 10, 110 delivers ACelectrical power to a load through an output 18 of the Inverter Module16. The system 10, 110 also automatically controls the charging anddischarging of the battery bank 25.

The DC to DC converter 14, 114 operates with its own current controlcircuit 15. It also communicates with the system controller 12 andreceives input signals from the fuel cell controller 22. Therefore, theterms “DC to DC Converter” and “DC to DC Buck and Boost Converter” canbe used to refer to not only the actual DC to DC Converter switchrelated hardware, but also to the current control circuit 15 and theintegration of these subsystems with the system controller 12 and thefuel cell (or photovoltaic array, or rectified AC, etc.) controller 22.

In both the series and parallel configurations, each DC bus 13 canoperate at its optimum voltage. In the relatively low voltage and highcurrent examples shown in FIGS. 2 and 3, the converter circuits 14, 114use power MOSFETs to charge and discharge inductors to transfer power toand from the DC power sources. Both the series and parallel powermanagement configurations 10, 110 are preferably configured to operatefully automatically between zero and full power throughput in any oftheir various operating modes. The transition between these modes can beseamless. Various possible modes include a first mode (e.g.,Startup/Shutdown Mode) in which the battery 25 is supplying all of thepower to the DC bus 13 and inverter 16; a second mode (e.g., NormalOperation Mode) in which the fuel cell 20 is supplying all of the powerto the inverter 16; a third mode (e.g., Recharge Mode) in which the fuelcell 20 is supplying all of the power to the inverter 16 and is alsocharging the battery 25; and a fourth mode (e.g., Transient Mode) inwhich the fuel cell 20 is supplying less than the total amount ofdesired power to the inverter 16 and the battery 25 is supplying thebalance of the power.

The series configured Power Management System 10, shown in FIG. 2, willnow be described in more detail. Referring specifically to FIG. 2, theterm “series” refers to the fact that the DC to DC Converter 14 islocated in series with the Fuel Cell 20 and the Inverter 16. The seriesconfiguration illustrated in FIG. 1 has an input to the inverter that isat the battery output node voltage. Unlike the conventional Hybrid Powerconfiguration, the series Power Manager 10 includes a Controller Module12 in addition to the DC to DC Converter Module 14 and the InverterModule 16. The combination of these modules can be referred to as aPower Manager or Power Management System 10.

The Controller Module (or System Controller) 12 manages both theInverter Module 16 and the Converter Module 14 to provide an integratedcontrol system. The primary functions of the Controller Module 12 are tocontrol the current drawn out of the fuel cell and to operate theInverter Module 16. The Controller 12 (in combination with the DC to DCConverter 14) controls the voltage of the variable voltage DC bus. TheControl Module 12 thereby provides coordinated control of the PowerManagement System 10. All of the modules, or subsystems, of the PowerManagement System 10 can be physically integrated together into a singlehardware package.

FIG. 3 shows one possible embodiment of a parallel Power ManagementSystem configuration 110. Referring to FIG. 3, in this parallel systemembodiment 110, the current control circuit 15 detects and controls theDC voltage on the buses for both the battery 25 and the power converter114. As shown, one of the primary advantages of the parallel system 110shown is that the voltage output from the fuel cell 20 goes directlyinto the inverter 16. Because only a relatively small amount of thepower is required to travel through the DC to DC converter (i.e., duringbattery charging or transient conditions), this configurationsignificantly reduces losses (such as switching losses) and improvesefficiency. This configuration also reduces noise and ripple currents.

Referring now to FIGS. 4, 5A, and 5B, three DC to DC Converter circuits114, 214, 314, respectively, have been developed to perform thefunctions of the invention. Of course, many other circuit arrangementscould be developed to perform the same functions and the invention istherefore not limited to any particular circuit arrangement. Circuit A,shown schematically in FIG. 4, provides a fully bi-directional buck andboost (and buck-boost) converter 114. Circuits B and C, illustrated inFIGS. 5A and 5B, respectively, are DC to DC converters 214, 314 with aboost capability only.

Referring to FIG. 4, Circuit A consists of an “H” switch bridge havingfour switch and diode pairs S_(A) and D_(A), S_(B) and D_(B), S_(C) andD_(C), and S_(D) and D_(D), coupled to the buck/boost inductor L_(BB).In this arrangement, the DC to DC converter 114 circuit allows both buckand boost current in both directions. This converter 114 provides acurrent-controlled type of converter that follows the current demandedfrom the fuel cell by the inverter (or other load). This circuitprovides excellent control of both bucking and boosting voltages with aminimum number of components.

Although FIG. 4 shows the DC to DC Converter 114 installed in serieswith a fuel cell 20 and inverter 16, such as in the Power ManagementSystem configuration shown in FIG. 2, because the converter can transferpower in either direction, the Circuit A converter 114 can also operatein the parallel configuration of FIG. 3. In the parallel PowerManagement System configuration 110 shown in FIG. 3, bi-directional buckand boost capabilities, such as those provided by this converter 114,are required. In the series configuration 10 illustrated in FIG. 2,however, only a single direction boost capability is required. Ofcourse, many other multi-source power systems are possible which wouldrequire the fully bi-directional buck and boost capabilities of theCircuit A converter 114 or a similar converter.

FIG. 6 is a graph illustrating the current flow over time through theinductor L_(BB) of Circuit A of FIG. 4. Referring to FIGS. 4 and 6,Circuit A operates as follows. When switch S_(A) is closed (on) andswitch S_(D) is closed (on), current flows from the fuel cell 20 throughthe inductor L_(BB) to ground. The circuit remains in this state untilthe inductor L_(BB) is charged to a threshold voltage. This state isshown as stage 1 in FIG. 6. A voltage (or current) monitoring circuit(not shown) monitors the voltage across (or current through) theinductor L_(BB.)

Once the inductor L_(BB) has been charged to the threshold voltage,switch S_(D) is opened (off) and switch S_(C) is closed (on), whileswitch S_(A) remains closed (on). This state is shown as stage 2 in FIG.6. During this stage, current is flowing from the fuel cell 20 to theinverter 16 and battery 25. If the voltage at the output of theconverter 114 (i.e., the input to the inverter 16) is raised (orboosted) above the voltage at the input of the converter 114 (i.e., theoutput terminal of the fuel cell 20), then the circuit is in the boostconfiguration. In the boost configuration, the slope of the line in FIG.6 representing stage 2 is downward and is proportional to the voltagedelta across the inductor L_(BB) (i.e., the difference between the inputand output voltages), and the current through the inductor L_(BB) isbeing dissipated.

If the voltage at the output of the converter 114 is lower than thevoltage at its input, then the circuit is in the buck configuration(i.e., voltage at the inverter input is reduced below the voltage at thefuel cell output). Current and power are still flowing from the fuelcell 20 to the inverter 16 and battery 25, but the slope of the line inFIG. 6 representing stage 2 will be upwards, again in proportion to thevoltage delta across the inductor L_(BB). In this state, the currentthrough the inductor continues to ramp up until switch S_(A) is opened.The converter 114 can be kept in the buck configuration simply bycycling switch S_(A) to allow the current to maintain a steady level.

In the boost configuration, after the energy in the inductor L_(BB) issubstantially dissipated, switch S_(A) is opened (off) and switchesS_(B) and S_(C) are closed (on). Closing switches S_(B) and S_(C)discharges the remaining energy in the inductor L_(BB) fast withoutstressing the two diodes D_(B) and D_(C). At the same time, however,some current is also flowing through the diodes D_(B) and D_(C). Whenthe current through (or voltage across) the inductor L_(BB) drops belowa threshold value (i.e., current less than 5A), switches S_(B) and S_(C)are opened (off), allowing the diodes D_(B) and D_(C) to complete thedischarge of the inductor L_(BB). Once the current through the inductorL_(BB) reaches zero, another cycle is started by closing switches S_(A)and S_(D.)

The inductor L_(BB) should be discharged after reaching the end of stage2 before beginning a new cycle (stage 1). A current from right to leftmay still be passing through the inductor L_(BB) at the end of stage 2.In other words, a reverse current may still exist in the inductor L_(BB)at this point. If switches S_(A) and S_(D) were simply turned back onand switches S_(C) and S_(B) turned off, a large forward current wouldbe generated from the fuel cell through the inductor L_(BB) to ground.These two oppositely directed currents could damage the diodes.Accordingly, the inductor L_(BB) is completely discharged before thenext boost cycle is started.

It should also be noted that the converter 114 of Circuit A maintains ahigh current level during stage 2. This high current level increases theRMS current and eliminates the need for generating a large current peak.The bi-directional aspect of the converter 114 of Circuit A allows theinductor L_(BB) to be discharged faster (i.e., during stages 3 and 4)between boost cycles (stages 1 and 2).

Power flow to the fuel cell 20, although not wanted in practice, is alsopossible using the bi-directional converter 114 of FIG. 4, and isexplained here for illustration. This reverse power flow is potentiallydesirable for rechargeable DC power sources. Power flow to the fuel cell20 (from right to left) is enabled in the boost configuration by closingswitch S_(C) and cycling S_(B). It is enabled in the buck configurationby cycling switch S_(C). A practical application of where power flowfrom the inverter/battery would be desirable is where the fuel cell isreplaced by a second storage device such as a battery. For example, abattery bank operating at nominally 24 VDC can be used to replace thefuel cell 20. In this circumstance, in the conventional direction (leftto right), the DC to DC converter 114 boosts that voltage to nominally48 VDC, which is the operating voltage of the original battery bank. Inthe opposite direction (right to left), the DC to DC converter 114 bucksthe voltage of the original battery bank to 24 VDC, for instance, torecharge the battery that replaced the fuel cell.

As noted briefly above, a novel method of substantially decreasing thepeak to RMS current ratio is also provided. This method is enabled, forinstance, by using four parallel gate (or switch) and diode pairs. Againreferring to FIGS. 4 and 6, the use of four parallel gate and diodepairs allows the converter 114 of Circuit A to reduce the current peakand tailor the shape of the current flowing through the inductor L_(BB)during each cycle. This is accomplished by operating pairs of switchesfor the first three stages of the cycle. During stage 1, the switchesS_(A) and S_(D) are on. During stage 2, the switches S_(A) and S_(C) areon. And, during stage 3, switches S_(B) and S_(C) are on. During thefourth and final stage (stage 4), current is conducted through diodesD_(B) and D_(C) only. Alternatively, the cycle can be constructed ofthree stages rather than four, with the first two stages being the sameas the first two stages described previously and with the third stagebeing that of conducting current through diodes D_(B) and D_(C) only. Inother words, the third stage of the four stage process could beeliminated, if desired, to create a three stage process.

“Boost only” DC to DC Converter circuits 214, 314 are shown in FIGS. 5Aand 5B, respectively. These converter circuits represent a significantimprovement to the conventional converter circuit design described inthe technical paper: Duarte, Claudio Manoel da Cunha and Ivo Barbi, “ANew Family of ZVS-PSW Active-Clamping DC-to-DC Boost Converters:Analysis, Design, and Experimentation” IEEE Transactions on PowerElectronics, Vol. 12, No. 5, September 1997, pp 824-831 (the “Duartepaper”) (see FIG. 5C).

Circuit B, shown in FIG. 5A, and Circuit C, shown in FIG. 5B, aredesigned to be zero-voltage switching (ZVS) pulse-width modulation (PWM)active-clamping DC to DC boost converters. The circuits are based on the“boost-buck-boost” circuit shown in FIG. 1(c) of the Duarte paper. Thegeneral circuit configuration shown in FIG. 1(c) of the Duarte paper isreproduced here as FIG. 5C. In each of the circuits shown in FIGS. 5A,5B, and 5C, power is transferred to the load during a boost stage, whilethe clamping action is performed during a buck-boost stage. Theconverter circuits of FIGS. 5A, 5B, and 5C differ from earlierconventional boost pulse-width modulation (PWM) converters because ofthe incorporation of an additional auxiliary switch (S₂), a resonantinductor (L_(R)), a resonant capacitor (C_(R1)) (which includes theoutput capacitance of the power switches), and a clamping capacitorC_(C). In each of these circuits, switch S₁ is the main switch. VoltageV_(S) is the input voltage and voltage V₀ is the output voltage.Inductor L_(R) and capacitor C_(R1) are the resonant circuit inductorand capacitor, respectively. Inductor L_(B) is the boost inductor.

Despite the similarities between these converter circuits, theconverters 214, 314 of Circuits B and C, respectively, each offer animproved design over that of the prior art circuit 414. Morespecifically, an additional capacitor is included in each of Circuits Band C to achieving soft switching and reliable current control for lowvoltage (e.g., less than about 100V) and high current (e.g., up toaround 150A) applications. The circuits developed and described byDuarte and Barbi are primarily aimed at high voltage (i.e., around300V˜400V), low current (i.e., around 5A) applications, and because ofthe low current involved, do not contemplate the need for additionalcapacitors for satisfactory switching. By recognizing and addressingthis need, the two converters 214, 314 of FIGS. 5A and 5B are able tooffer improved performance over prior art DC to DC converter circuits,such as the one 414 illustrated in FIG. 5C, in low voltage/high currentapplications.

Referring to FIG. 5A, the DC to DC converter 214 of Circuit B includesthe addition of a capacitor C_(R2) on the output side of the resonantcircuit across the auxilliary switch S₂. The second resonant capacitorC_(R2) is beneficial in low voltage and high current applications suchas fuel cell and battery systems because it substantially reduces outputDC current ripple and switching losses and provides better control forcombined low voltage/high current applications.

The principle of operation of Circuit B is as follows. The inductance inthe boost inductor L_(B) is assumed to be large enough for the inductorto act as a current source (I_(S)). The clamping capacitor C_(C) isselected to have a large capacitance so that the voltage V_(C) acrossthis capacitor can be considered a constant. The main and auxiliaryswitches S₁ and S₂, respectively, are switched in a complementary way.The main switch S₁ is turned off at time (t)=t_(o), when the switchingperiod starts.

Before time t_(o), the main switch S₁ is on and the auxiliary switch S₂is off. When S₁ is turned off at time t_(o), the resonant capacitorC_(R1) is linearly charged, by the boost inductor current I_(S), to abase voltage V₀. Due to the presence of the resonant capacitor C_(R1),the main switch S₁ is turned off with no switching loss. When thevoltage V_(CR1) reaches the voltage V₀, the boost diode D_(B) beginsconducting. The current through the resonant inductor L_(R) and theresonant capacitor C_(R1) then evolves in a resonant way, and theresonant capacitor voltage V_(CR1) rises from the base voltage V₀ up toa increased voltage equal to V_(Cc)+V₀. At that point, the voltages areclamped. As the capacitor voltage V_(CR1) becomes equal to V_(Cc)+V₀,the voltage across the auxiliary switch S₂ is zero, and the switch S₂turns on with a no loss zero-voltage switching (ZVS). The resonantinductor L_(R) current then ramps down until it reaches zero, when itchanges its direction and rises again.

This stage ends when the auxiliary switch S₂ is turned off with zerovolts on the switch S₂ due to the presence of the added capacitor C_(R2)at time t₃ (t=t₃). The voltage across the resonant capacitor C_(R1)falls, due to resonance between inductor L_(R) and the first or secondresonant capacitor C_(R1) or C_(R2), until it reaches zero at time t₄(t=t₄). In stage 5, the main switch S₁ is turned on without anyswitching losses (ZVS), because the first resonant capacitor voltageV_(CR1) became null. During this stage, the current through the resonantinductor L_(R) changes its polarity and ramps up to reach the boostinductor current I_(S). At time t₅ (t=t₅), the diode D_(B) becomesreversibly biased and power is not transferred to the load. This stageends when the main switch S₁ is turned off at the end of the firstswitching cycle to start the next switching cycle.

Circuit B offers an improvement over the prior art by splitting itsresonant circuit capacitance between two capacitors C_(R1) and C_(R2) toallow zero or low voltage switching by both switches S₁ and S₂. Moreparticularly, the second resonant capacitor C_(R2) acts as part of theresonant control circuit that includes the resonant inductor L_(R) andthe first resonant capacitor C_(R1). The oscillation that occurs whenthe main switch S₁ is switched on and off can be neutralized using thesecond capacitor C_(R2) by selectively switching the auxiliary switchS₂. When the second capacitor C_(R2) is substantially smaller than thefirst capacitor C_(R1), oscillation control can be provided at lowvoltage. By dividing the capacitance between the two resonant circuitcapacitors C_(R1) and C_(R2), the voltage on switches S₁ and S₂ can beramped down to allow zero voltage switching. Furthermore, because thecapacitors store the voltages with no voltage losses, they provideefficient power transfer between the input and the output nodes of theconverter 214.

Referring now to FIG. 5B, the DC to DC converter 314 of Circuit C isanother boost converter embodiment capable of use in the PowerManagement System 10 of FIG. 2. In this converter 314, an inputcapacitor C₁ has been added to the input side of the resonant circuit ofthe prior art converter 414 of FIG. 5C. Without the capacitor C₁, thevoltage at node A dips in high current applications, and large voltageswings occur at that node. These voltage swings cause a large voltagestress on switch S₁ and result in an inability to achieve reliablecontrol of the current through the resonant inductor L_(R.)

With the addition of capacitor C₁ in the circuit, the voltage at node Acan be held steady during switching. Because the input capacitor C₁maintains a constant voltage at node A, the voltage stress on the mainswitch S₁ is reduced. Specifically, by dampening the voltage swings atnode A, the input capacitor C₁ filters out high voltage swings and keepsthem from affecting the switch S₁. This steady voltage at node A helpsachieve soft switching and good current control.

Having described and illustrated the principles of the invention invarious preferred embodiments thereof, it should be apparent that thevarious implementations of the invention described above can be modifiedin arrangement and detail without departing from such principles. Weclaim all modifications and variations coming within the spirit andscope of the following claims.

What is claimed is:
 1. A power management system, comprising: aplurality of independent DC power sources connected to a DC bus, whereinat least one of the independent power sources comprises a rechargeableDC power source for supplying power to and receiving power from the DCbus, and wherein at least one of the DC power sources is a fuel cell; aDC to DC converter connected between an output of the rechargeable DCpower source and the DC bus in parallel with the fuel cell; and a systemcontroller for managing power transfer between the multiple independentDC power sources and the DC bus.
 2. A power management system accordingto claim 1, wherein the system controller is configured to control thecurrent supplied from each of the power sources to the DC bus and fromthe DC bus to each of the power sources.
 3. A power management systemaccording to claim 1, further comprising an inverter for converting DCpower from the independent DC power sources into AC power.
 4. A powermanagement system according to claim 1, wherein the fuel cell comprisesa fuel cell controller.
 5. A power management system according to claim4, wherein the system controller communicates with the fuel cellcontroller to control the amount of power supplied by the fuel cell tothe DC bus.
 6. A power management system according to claim 1, whereinthe DC to DC converter further comprises a converter control circuitthat communicates with the system controller to control an amount ofpower transferred between the DC to DC converter and the DC bus.
 7. Apower management system according to claim 1, further comprising aninverter, wherein the DC to DC converter is arranged in series betweenthe rechargeable DC power source and the inverter.
 8. A power managementsystem according to claim 1, further comprising an inverter, wherein theDC to DC converter is arranged in parallel with the fuel cell and theinverter.
 9. A method for managing multiple independent DC powersources, comprising: arranging a plurality of independent DC powersources in communication with a DC bus, wherein at least one of the DCpower sources comprises a fuel cell; converting a voltage from one ormore of the independent DC power sources to conform to a voltage of theDC bus, wherein converting a voltage from one or more of the independentDC power sources to conform to a voltage of the DC bus comprisesarranging a DC to DC converter in series between one or more of theindependent DC power sources and the DC bus and in parallel with thefuel cell and the DC bus; and controlling the amount of power that theDC bus supplies to or receives from one or more of the independent powersources.
 10. A method of managing multiple independent DC power sourcesaccording to claim 9, wherein controlling the amount of power that theDC bus supplies to or receives from one or more of the independent powersources comprises integrating between a plurality of modes wherein eachof the DC power sources is either supplying some or all of the requiredpower, being recharged by one or more of the other DC power sources, orbeing isolated from the DC bus.
 11. A method of managing multipleindependent DC power sources according to claim 9, wherein converting avoltage from one or more of the independent DC power sources to conformto a voltage of the DC bus comprises arranging a DC to DC converter inseries between a rechargeable DC power source and the DC bus.
 12. Amethod of managing multiple independent DC power sources according toclaim 11, wherein the DC to DC converter is arranged in parallel withthe fuel cell and an inverter.
 13. A method of managing multipleindependent DC power sources according to claim 9, further comprisingsupplying the DC power from the DC bus to an inverter for conversioninto AC power to enable the provision of AC power to a load.